Reverse conducting insulated gate bipolar transistor

ABSTRACT

A semiconductor includes a drift zone of a first conductivity type arranged between a first side and a second side of a semiconductor body. The semiconductor device further includes a first region of the first conductivity type and a second region of a second conductivity type subsequently arranged along a first direction parallel to the second side. The semiconductor device further includes an electrode at the second side adjoining the first and second regions. The semiconductor device further includes a third region of the second conductivity type arranged between the drift zone and the first region. The third region is spaced apart from the second region and from the second side.

BACKGROUND

Reverse conducting semiconductor devices, e.g. Reverse ConductingInsulated Gate Bipolar Transistors (RC IGBTs), allow to operate in atransistor mode, e.g. IGBT mode, and in a diode mode, e.g. freewheelingdiode mode, by using a same active area in a semiconductor body. Duringdesign of reverse conducting semiconductor devices, trade-offs betweenelectrical characteristics in diode and transistor modes have to beconsidered, e.g. trade-offs between forward characteristics, robustnessand softness.

It is desirable to design reverse conducting semiconductor devicesincluding an improved trade-off between the electrical characteristicsin a diode mode and in a transistor mode.

SUMMARY

According to an embodiment of a semiconductor device, the semiconductorincludes a drift zone of a first conductivity type arranged between afirst side and a second side of a semiconductor body. The semiconductordevice further includes a first region of the first conductivity typeand a second region of a second conductivity type subsequently arrangedalong a first direction parallel to the second side. The semiconductordevice further includes an electrode at the second side adjoining thefirst and second regions. The semiconductor device further includes athird region of the second conductivity type arranged between the driftzone and the first region. The third region is spaced apart from thesecond region and from the second side.

According to an embodiment of a RC IGBT, the RC IGBT includes a driftzone of a first conductivity type arranged between an emitter side and acollector side of a semiconductor body. The RC IGBT further includes afirst emitter region of the first conductivity type and a second emitterregion of a second conductivity type subsequently arranged along a firstdirection parallel to the second side. The RC IGBT further includes anelectrode at the second side adjoining the first and second emitterregions. The RC IGBT further includes a third region of the secondconductivity type arranged between the drift zone and the first region.The third region is spaced apart from the second emitter region and fromthe second side.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of the specification. The drawings illustrateembodiments of the present invention and together with the descriptionserve to explain principles of the invention. Other embodiments of theinvention and many of the intended advantages will be readilyappreciated as they become better understood by reference to thefollowing detailed description. The elements of the drawings are notnecessarily to scale relative to each other. Like reference numeralsdesignate corresponding similar parts.

FIG. 1A is a schematic cross-section of an RC IGBT including a floatingp⁻-type semiconductor zone for improving the trade-off between theelectrical characteristics in a diode mode and in a transistor mode.

FIG. 1B is a schematic illustration of different current voltagecharacteristics of an RC IGBT.

FIG. 1C illustrates one example of an arrangement of semiconductor zonesconstituting the IGBT cells illustrated in FIG. 1A.

FIGS. 2 to 6 are schematic cross-sections of RC IGBTs includingdifferent designs of a floating p⁻-type semiconductor zone for improvingthe trade-off between the electrical characteristics in a diode mode andin a transistor mode.

FIGS. 7A to 7C illustrate examples of designs of p⁺-type regions actingas an emitter in an IGBT mode and n⁺-type regions acting as an emitterin a diode mode of an RC IGBT.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described as part of one embodiment can be used inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention includes such modifications andvariations. The examples are described using specific language whichshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements or manufacturing processes are designated bythe same references in the different drawings if not stated otherwise.

As employed in the specification, the term “electrically coupled” is notmeant to mean that the elements must be directly coupled together.Instead, intervening elements may be provided between the “electricallycoupled” elements. As an example, none, part, or all of the interveningelement(s) may be controllable to provide a low-ohmic connection and, atanother time, a non-low-ohmic connection between the “electricallycoupled” elements. The term “electrically connected” intends to describea low-ohmic electric connection between the elements electricallyconnected together, e.g., a connection via a metal and/or highly dopedsemiconductor.

Some Figures refer to relative doping concentrations by indicating “⁻”or “⁺” next to the doping type. For example, “n⁻” means a dopingconcentration which is less than the doping concentration of an“n”-doping region while an “n⁺”-doping region has a larger dopingconcentration than the “n”-doping region. Doping regions of the samerelative doping concentration may or may not have the same absolutedoping concentration. For example, two different n⁺-doped regions canhave different absolute doping concentrations. The same applies, forexample, to an n⁻-doped and a p⁺-doped region. In the embodimentsdescribed below, a conductivity type of the illustrated semiconductorregions is denoted n-type or p-type, in more detail one of n⁻-type,n-type, n⁺-type, p⁻-type, p-type and p⁺-type. In each of the illustratedembodiments, the conductivity type of the illustrated semiconductorregions may be vice versa. In other words, in an alternative embodimentto any one of the embodiments described below, an illustrated p-typeregion may be n-type and an illustrated n-type region may be p-type.

Terms such as “first”, “second”, and the like, are used to describevarious structures, elements, regions, sections, etc. and are notintended to be limiting. Like terms refer to like elements throughoutthe description.

The terms “having”, “containing”, “including”, “comprising” and the likeare open and the terms indicate the presence of stated elements orfeatures, but not preclude additional elements or features. The articles“a”, “an” and “the” are intended to include the plural as well as thesingular, unless the context clearly indicates otherwise.

FIG. 1A illustrates a cross-section of a part of an RC IGBT device 100according to an embodiment. The semiconductor device 100 includes asemiconductor body 101, e.g. a semiconductor substrate including none,one or a plurality of semiconductor layers thereon. As an example, thesemiconductor substrate includes silicon.

An emitter side 102 of the semiconductor body 101, e.g. a first side,includes IGBT cells 104. The IGBT cells 104 are illustrated in asimplified manner and include any suitable arrangement of dielectricmaterial(s), semiconductor materials(s) and conductive material(s)configured as emitter and gate of an IGBT.

Between the emitter side 102 and a collector side 105, e.g. a secondside, an type drift zone 106 is arranged. The n⁻-type drift zone 106 maybe a part of the semiconductor body 101.

An n⁺-type region 107 and a p⁺-type region 108 are subsequently arrangedalong a lateral direction×parallel to the collector side 105. Then⁺-type region 107 acts as an emitter in a diode mode of the RC IGBT100. The p⁺-type region 108 acts as an emitter in an IGBT mode of the RCIGBT 100. An electrode 111 including a conductive material, e.g. ametal, a metal alloy or a combination thereof, is electrically coupledto both the p⁺-type region 108 and the n⁺-type region 107.

A p⁻-type semiconductor region 109 is arranged above the n⁺-type emitterregion 107 and at least partly covers the n⁺-type emitter region 107.According to the embodiment illustrated in FIG. 1, the p⁻-typesemiconductor region 109 covers a part of the n⁺-type emitter region 107and is absent in an area above the p⁺-type emitter region 108. Accordingto another embodiment, the p⁻-type semiconductor region 109 fully coversthe n⁺-type emitter region 107 and is absent in an area above thep⁺-type emitter region 108. According to yet another embodiment, thep⁻-type semiconductor region 109 at least partly covers the n⁺-typeemitter region 107 and partly covers an area above the p⁺-type emitterregion 108. The coverage ratio of the n⁺-type emitter region 107 andp⁺-type emitter region 108 may vary with regard to an arrangement of aplurality of n⁺-type emitter regions 107 and p⁺-type emitter regions108. The p⁻-type semiconductor region 109 and the p⁺-type emitter region108 are spaced apart from each other and a shortest distance between thep⁻-type semiconductor region 109 and the p⁺-type emitter region 180 istermed d and satisfies d>0 μm. In other words, the p⁻-type semiconductorregion 109 is electrically floating. As an example, the p⁻-typesemiconductor region 109 may be fully surrounded by n-type semiconductormaterial.

An n-type field stop zone 110 is arranged between the n⁻-type drift zone106 and the p⁻-type semiconductor region 109. The n-type field stop zone110 adjoins the p⁻-type semiconductor region 109 and covers the n⁺-typeregion 107 and the p⁺-type region 108.

The p⁻-type semiconductor region 109 allows to improve a trade-offbetween the electrical characteristics in a diode mode and an IGBT mode.In an RC IGBT without the p⁻-type semiconductor region 109, injection ofholes from the p⁺-type emitter region 108 into the n⁻-type drift zone106, i.e. onset of bipolar current flow between the emitter side 102 andthe collector side 105, requires a forward-biased junction between thep⁺-type emitter region 108 and the n⁻-type drift zone 106. This junctionmay be forward-biased by a resistive voltage drop by electrons flowingthrough the n⁻-type drift zone 106 to the n⁺-type emitter region 107. Adecrease of lateral dimensions of the p⁺-type emitter region 108 maylead to an increase of an electron current density required toforward-bias the junction between the p⁺-type emitter region 108 and then⁻-type drift zone 106. This current/voltage (IV) behavior may beaccompanied by a characteristic in the IV behavior called “nose” hereinthat is illustrated as curve 230 in the schematic graph of FIG. 1B.Curve 230 illustrated in FIG. 1B is one example of an IV characteristicfor an RC IGBT including a pattern of successively arranged p⁺-typeemitter regions and n⁺-type emitter regions with typical lateraldimensions of less than 5 times the thickness of the drift region andlacking the p⁻-type semiconductor region 109. A unipolar current flowsup to a current I_J associated with a voltage U_J. For currents largerthan I_J, the p⁺-type emitter region gets forward-biased and triggersbipolar current flow. This leads to a snap back of the voltage up to avalue U_JB<U_J. A counter measure for reducing or avoiding the nose orsnap back while maintaining the dimensions of the p⁺-type and then⁺-type emitter regions lies in the arrangement of the p⁻-typesemiconductor region 109. Emission of holes from the p⁻-typesemiconductor region 109 occurs at low current densities, i.e. atcurrents I<I_J, by operating the pn junction between the floatingp⁻-type semiconductor region 109 and the surrounding n-typesemiconductor material in avalanche or due to punch-through from thefloating p⁻-type semiconductor region 109 to the p⁺-type emitter region108. Thus, the arrangement of the p⁻-type semiconductor region 109allows to prevent or reduce the so-called nose characteristic in the IVcurve of the RC IGBT in the IGBT mode. As an example, arrangement of thep⁻-type semiconductor region 109 may lead to an IV curve as illustratedby curve 240 in FIG. 1B.

FIG. 1C illustrates one example of arrangement of semiconductor zonesconstituting the IGBT cells 104 illustrated in FIG. 1A. The IGBT cells104 include a p-type body region 150 in the semiconductor body 101. Thep-type body region 150 adjoins the emitter side 102. An n⁺-type sourceregion 160 is formed in the p-type body region 150. The p-type bodyregion 150 and the n⁺-type source region 160 are electrically coupled toa contact structure 185 at the emitter side 102. Between the p-type bodyregion 150 and the contact structure 185 a p⁺-type contact zone may bearranged (not illustrated in FIG. 1C).

The conductivity in a channel region 180 formed in the p-type bodyregion 150 at the emitter side 102 can be controlled via a voltageapplied to a gate electrode 170. A gate dielectric 172 is arrangedbetween the gate electrode 170 and the channel region 180.

The arrangement of semiconductor zones constituting the IGBT cells 104as illustrated in FIG. 1C is one example. Other arrangements maysubstitute the specific arrangement shown in FIG. 1C.

FIG. 2 illustrates a cross-section of a part of an RC IGBT device 200according to another embodiment. Similar to the RC IGBT 100 illustratedin FIG. 1A, the RC IGBT device 200 includes a semiconductor body 201having an emitter side 202 and a collector side 205, IGBT cells 204, ann⁻-type drift zone 206, an n⁺-type region 207 acting as an emitter in adiode mode of the RC IGBT 200, a p⁺-type region 208 acting as an emitterin an IGBT mode of the RC IGBT 200, an n-type field stop zone 210 and anelectrode 211.

The RC IGBT device 200 further includes a continuous p⁻-typesemiconductor region 209 fully covering both the n⁺-type region 207 andthe p⁺-type region 208. As an example, the p⁻-type semiconductor region209 may be formed without a mask or by using a mask common to formationof the n-type field stop zone 210 and the p⁻-type semiconductor region209, thereby contributing to a reduction of manufacturing costs. Asemiconductor region between the p⁻-type semiconductor region 209 andthe emitter regions 208, 209 may include a doping similar to the n⁻-typedrift zone 206. When measuring the forward characteristic of the RC IGBTdevice 200 in a diode mode, depending on the doping level of the p⁻-typesemiconductor region 209 a nose may appear in the IV characteristic dueto a reverse operation of the junction between the n-type field stopzone 210 and the p⁻-type semiconductor region 209. The nosecharacteristic may be adjusted by a thickness and doping of the p⁻-typesemiconductor region 209, for example. As an example, assuming athickness in a range of 100 nm to 500 nm and a doping in a range of 10¹⁴cm⁻³ to 10¹⁵ cm⁻³, a punch-through voltage amounts to values as small as0.8 mV to 0.2 V resulting in a nose characteristic that is negligible oralmost negligible. Further it is to be noted that flooding the p⁻-typesemiconductor region 209 with excess carriers occurs after onset offorward diode operation.

FIG. 3 illustrates a cross-section of a part of an RC IGBT device 300according to another embodiment. Similar to the RC IGBT 200 illustratedin FIG. 2, the RC IGBT device 300 includes a semiconductor body 301having an emitter side 302 and a collector side 305, IGBT cells 304, ann⁻-type drift zone 306, an n⁺-type region 307 acting as an emitter in adiode mode of the RC IGBT 300, a p⁺-type region 308 acting as an emitterin an IGBT mode of the RC IGBT 300, an electrode 311 and an n-type fieldstop zone 310.

The RC IGBT device 300 further includes a p⁻-type semiconductor region309 partly covering the n⁺-type region 307. The p⁻-type semiconductorregion 309 is absent, i.e. not formed, in an area above the p⁺-typeregion 308. Opening the p⁻-type semiconductor region 309 above thep⁺-type region 308 allows to avoid or reduce a nose in the IVcharacteristic of the diode mode that may appear in the embodiment asillustrated in FIG. 2 as described above.

FIG. 4 illustrates a cross-section of a part of an RC IGBT device 400according to another embodiment. Similar to the RC IGBT 300 illustratedin FIG. 3, the RC IGBT device 400 includes a semiconductor body 401having an emitter side 402 and collector side 405, IGBT cells 404, ann⁻-type drift zone 406, an n⁺-type region 407 acting as an emitter in adiode mode of the RC IGBT 400, a p⁺-type region 408 acting as an emitterin an IGBT mode of the RC IGBT 400, an electrode 411 and an n-type fieldstop zone 410.

The RC IGBT device 400 further includes a p⁻-type semiconductor region409 fully covering the n⁺-type region 407 and partly covering thep⁺-type region 408. Hence, a rate of coverage of both the p⁺-type region408 and the n⁺-type region 407 is larger in the RC IGBT device 400 thanin the RC IGBT device 300 illustrated in FIG. 3. Since an overall areaof the p⁻-type semiconductor region 409 may be adjusted independent ofthe overall area of the n⁺-type region 407, the overall area of thep⁻-type semiconductor region 409 may be substantially larger than anoverall area of the p⁺-type region 408. This allows avoiding a nose inthe IV characteristic of the IGBT mode. Further, since the p⁻-typesemiconductor region 409 is absent in at least a part of the area abovethe p⁺-type region 408 acting as an emitter in an IGBT mode, a nose inthe IV characteristic of the diode mode may be prevented. When aplurality of p⁺-type regions 408 are arranged at the collector side 405,a first part of the p⁺-type regions 408 may be fully covered by thep⁻-type semiconductor region 409 provided that a second part of thep⁺-type regions 408 are partly covered or not covered.

FIG. 5 illustrates a cross-section of a part of an RC IGBT device 500according to another embodiment. Similar to the RC IGBT 300 illustratedin FIG. 3, the RC IGBT device 500 includes a semiconductor body 501having an emitter side 502 and collector side 505, IGBT cells 504, ann⁻-type drift zone 506, an n⁺-type region 507 acting as an emitter in adiode mode of the RC IGBT 500, a p⁺-type region 508 acting as an emitterin an IGBT mode of the RC IGBT 500, an electrode 511 and an n-type fieldstop zone 510.

The RC IGBT device 500 further includes a p⁻-type semiconductor region509 partly covering the n⁺-type region 507. The p⁻-type semiconductorregion 509 is absent, i.e. not formed, in an area above the p⁺-typeregion 508. Opening the p⁻-type semiconductor region 509 above thep⁺-type region 508 allows to avoid or reduce a nose in the IVcharacteristic of the diode mode that may appear in the embodiment asillustrated in FIG. 2 as described above. A bottom side of the p⁻-typesemiconductor region 509 adjoins the n⁺-type region 507 acting as anemitter in a diode mode of the RC IGBT 500. The embodiment illustratedin FIG. 5 may be beneficial with regard to manufacturing aspects sincethe p⁻-type semiconductor region 509 may be formed by comparativelyshallow ion implantation.

FIG. 6 illustrates a cross-section of a part of an RC IGBT device 600according to another embodiment. Similar to the RC IGBT 300 illustratedin FIG. 3, the RC IGBT device 600 includes a semiconductor body 601having an emitter side 602 and collector side 605, IGBT cells 604, ann⁻-type drift zone 606, an n⁺-type region 607 acting as an emitter in adiode mode of the RC IGBT 600, a p⁺-type region 608 acting as an emitterin an IGBT mode of the RC IGBT 600, an electrode 611 and an n-type fieldstop zone 610.

The RC IGBT device 600 further includes a p⁻-type semiconductor region609 partly covering the n⁺-type region 607. The p⁻-type semiconductorregion 609 is absent, i.e. not formed, in an area above the p⁺-typeregion 608. Opening the p⁻-type semiconductor region 609 above thep⁺-type region 608 allows avoiding or reducing a nose in the IVcharacteristic of the diode mode that may appear in an embodiment asillustrated in FIG. 2 as described above. The p⁻-type semiconductorregion 609 is embedded in the n-type field stop zone 610. A bottom sideof the n-type field stop zone 610 adjoins both the p⁺-type region 608acting as an emitter in an IGBT mode of the RC IGBT 600 and the n⁺-typeregion 607 acting as an emitter in a diode mode of the RC IGBT 600. Theother examples for the design of the p⁻-type semiconductor region whichare described above can be embedded in the field stop layer according toFIG. 6.

A typical design for the n⁺-type region 107 of FIG. 1A (as well as then⁺-type regions 207, 307, 407, 507, 607 illustrated in FIGS. 2 to 6) andthe p⁺-type region 108 of FIG. 1A (as well as the p⁺-type regions 208,308, 408, 508, 608 illustrated in FIGS. 2 to 6) is a stripe design asschematically illustrated in FIG. 7A with reference to a cut line A-A′of FIG. 1A. A further example for a design of the n⁺-type region 107 ofFIG. 1A (as well as the n⁺-type regions 207, 307, 407, 507, 607illustrated in FIGS. 2 to 6) and the p⁺-type region 108 of FIG. 1A (aswell as the p⁺-type regions 208, 308, 408, 508, 608 illustrated in FIGS.2 to 6) is a design in which each p⁺-type region 108 is surrounded by ann⁺-type region or vice versa as illustrated in FIGS. 7B and 7C. In sucha design, the n⁺-type regions 107 of FIG. 7B and/or the p⁺-type regions108 of FIG. 7C may be of square, rectangular, circular shape or acombination thereof.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A reverse conducting semiconductor device,comprising: a drift zone of a first conductivity type arranged between afirst side and a second side of a semiconductor body; a first region ofthe first conductivity type and a second region of a second conductivitytype subsequently arranged along a first direction parallel to thesecond side; an electrode at the second side adjoining the first andsecond regions; a third region of the second conductivity type arrangedbetween the drift zone and the first region, and wherein the thirdregion is spaced apart from the second region and from the second side,and from the first region by a fourth semiconductor region of the firstconductivity type, wherein a doping concentration of the drift zone andthe fourth semiconductor region is less than a doping concentration ofthe first region.
 2. The reverse conducting semiconductor device ofclaim 1, wherein the reverse conducting semiconductor device is areverse conducting IGBT, the first side is an emitter side and thesecond side is a collector side.
 3. The reverse conducting semiconductordevice of claim 1, wherein an average concentration of doping of thefourth semiconductor region along a direction perpendicular to thesecond side is in a range of 10¹⁴ cm-3 to 10¹⁶ cm-3.
 4. The reverseconducting semiconductor device of claim 1, wherein the thirdsemiconductor region is an electrically floating semiconductor region.5. The reverse conducting semiconductor device of claim 1, wherein thethird semiconductor region is fully surrounded by semiconductor materialof the first conductivity type.
 6. The reverse conducting semiconductordevice of claim 1, wherein the third semiconductor region covers thefirst and second semiconductor regions and has a thickness in a range of50 nm to 5 μm.
 7. The reverse conducting semiconductor device of claim1, wherein an average doping concentration of the third semiconductorregion along a vertical direction perpendicular to the second side is ina range of 10¹⁴ cm-3 to 10¹⁶ cm-3.
 8. The reverse conductingsemiconductor device of claim 1, wherein the third semiconductor regionfully covers the first and second semiconductor regions.
 9. The reverseconducting semiconductor device of claim 1, wherein the thirdsemiconductor region covers at least partly the first semiconductorregion and is absent in an area above the second semiconductor region.10. The reverse conducting semiconductor device of claim 1, wherein thethird semiconductor region fully covers the first semiconductor regionand partly covers the second semiconductor region.
 11. The reverseconducting semiconductor device of claim 1, wherein the thirdsemiconductor region is contiguous and includes apertures, wherein anaperture area ratio is in a range of 0% to 90%.
 12. The reverseconducting semiconductor device of claim 1, wherein the thirdsemiconductor region adjoins the first semiconductor region.
 13. Thereverse conducting semiconductor device of claim 1, further comprising aplurality of the first and second semiconductor regions alternatelyarranged along the lateral direction, wherein a maximum lateraldimension of each one of the second semiconductor regions is smallerthan 5 times of the thickness of the drift zone.
 14. The reverseconducting semiconductor device of claim 1, wherein a thickness of thefourth semiconductor region is in a range of 50 nm to 5 μm.
 15. Thereverse conducting semiconductor device of claim 1, further comprising afield stop zone of the first conductivity type between the drift zoneand the third semiconductor region, wherein the field stop zone has anaverage doping concentration along a vertical direction perpendicular tothe second side that is larger than an average doping concentration ofthe drift zone along the vertical direction.
 16. The reverse conductingsemiconductor device of claim 15, wherein the third semiconductor regionis embedded in the field stop zone.
 17. The reverse conductingsemiconductor device of claim 15, wherein the field stop zone adjoinsthe second semiconductor region.
 18. A reverse conducting insulated gatebipolar transistor, comprising: a drift zone of a first conductivitytype arranged between an emitter side and a collector side of asemiconductor body; a first emitter region of the first conductivitytype and a second emitter region of a second conductivity typesubsequently arranged along a first direction parallel to thecollector-side; an electrode at the collector side adjoining the firstand second emitter regions; a third region of the second conductivitytype arranged between the drift zone and the first region, and whereinthe third region is spaced apart from the second region and from thesecond side, and from the first region by a fourth semiconductor regionof the first conductivity type, wherein a doping concentration of thedrift zone and the fourth semiconductor region is less than a dopingconcentration of the first region.
 19. The reverse conducting insulatedgate bipolar transistor of claim 18, wherein the third semiconductorregion is an electrically floating semiconductor region.
 20. The reverseconducting insulated gate bipolar transistor of claim 18, wherein thethird semiconductor region is fully surrounded by semiconductor materialof the first conductivity type.
 21. The reverse conducting semiconductordevice of claim 18, wherein a thickness of the fourth semiconductorregion is in a range of 50 nm to 5 μm.